1. Field of the Invention
The present invention relates generally to ink-jet hard copy apparatus, and, more specifically, to methods and apparatus for the use of electrostatic devices for detection of ink drop characteristics and printing with correction for offsets.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller 102 (usually a microprocessor-controlled printed circuit board) connected by appropriate cabling to a computer (not shown). Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text is created. A carriage 109, mounted on a slider 111, scans the print medium. An encoder 113, 114 subsystem is provided for keeping track of the position of the carriage 109 at any given time. A set of ink-jet pens, or print cartridges, 115.sub.x (where the letter is a color designation, e.g., cyan (C), magenta(M), yellow (Y), black (K), red (R), blue (B), green (G), or a fixer chemical (F)) are releasably mounted in the carriage 109 for easy access. In pen-type hard copy apparatus, separate, replaceable or refillable, ink reservoirs 117.sub.x are located within the housing 103 and appropriately coupled to the pen set 115 via ink conduits 119. Once a printed page is completed, the print medium is ejected onto an output tray 121. Printing is accomplished on the print medium as it transits a print zone 123.
"A simplistic schematic of a swath-scanning ink-jet pen 115 is shown in FIG. 2 (PRIOR ART). The body 210 of the pen 115 generally contains an ink accumulator and regulator mechanism 212. The internal accumulator and regulator are fluidically coupled 119 (FIG. 1 only) to an off-axis ink reservoir 117.sub.x in any known manner to the state of the art. The printhead 214 element includes an appropriate electrical connections 220 (such as a tape automated bonding flex tape) for transmitting signals to and from the printhead. Columns of individual nozzles 217 form an addressable firing array 216. The typical state of the art scanning pen printhead may have two or more columns with more than one-hundred nozzles per column, each nozzle having a diameter of about 1/300th inch or less. Multi-color pens having the nozzle array 216 is subdivided into discrete subsets, known as "primitives" are also known in the art. In a thermal ink-jet pen, the drop generator includes a heater resistor subjacent each nozzle which on command superheats local ink to a cavitation point such that an ink bubble's expansion and collapse ejects a droplet from the associated nozzle 217. In commercially available products, piezoelectric and wave generating element techniques are also used to fire the ink drops. Degradation or complete failure of the drop generator elements cause drop volume variation, trajectory error, or misprints, referred to generically as "artifacts," and thus affect print quality.
In essence, the ink-jet printing process involves digitized dot-matrix manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent print media. [For convenience of describing the ink-jet technology and the present invention hereinafter, all types of print media are referred to simply as "paper," all compositions of colorants are referred to simply as "ink," and all types of hard copy apparatus are referred to simply as a "printer." No limitation on the scope of invention is intended nor should any be implied.] Each column or selected subset of nozzles selectively fires ink droplets (typically each being only a few picoliters in liquid volume, having a nominal diameter of only about ten in flight and forming a dot of approximately forty .mu.m on the paper) that create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned. The pen scanning axis is the x-axis, the paper path is the y-axis and the ink drop firing direction is the z-axis; related linear offsets are referred to as delta-x, delta-y and delta-z, respectively, and rotational offsets are referred to as theta-x (printhead planar pitch), theta-y (roll) and theta-z (yaw). A given nozzle of the printhead is used to address a given matrix column print position on the paper (referred to as a picture element, or "pixel").
Horizontal positions, matrix pixel rows, on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of tens of thousands of dots. The paper is stepped to permit a series of contiguous swaths. Complex digital dot matrix manipulation is used to render alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Page-wide ink-jet printheads are also contemplated and are adaptable to the present invention.
As can now be recognized, the seemingly simple process of creating a computer print by scanning a plurality of printheads while actively firing minute ink droplets across a z-axis gap onto a sheet of paper as a digital dot matrix of organized pixels in order to form sophisticated graphics and photographs is actually a highly complex process. The reduction of visible artifacts in the print is a constant concern of the system designer.
A variety of techniques have been used over the years since the inception of ink-jet printing to ensure appropriate dot placement. In U.S. Pat. No. 4,794,411, filed in 1987 by Taub et al., a THERMAL INK-JET HEAD STRUCTURE WITH ORIFICE OFFSET FROM RESISTOR methodology teaches a controlling of misdirection of fired drops by proper nozzle design. In U.S. Pat. No. 4,922,268, filed in 1989 by Osborne, a PIEZOELECTRIC DETECTOR FOR DROP POSITION DETERMINATION IN MULTI-PEN THERMAL INK JET PEN PRINTING SYSTEMS teaches a methodology for mapping the positions of nozzles with respect to a pattern of openings in the detector [U.S. Pat. No. 5,036,340 filed in 1990 by Osborne is a continuation-in-part of '268.] In U.S. Pat. No. 4,922,270 filed simultaneously with Osborne by Cobbs et al., an optical or piezoelectric or electrostatic phase plate detector through which a drop is fired and measurements are used for INTER PEN OFFSET DETERMINATION AND COMPENSATION IN MULTI-PEN THERMAL INK JET PEN PRINTING SYSTEMS [U.S. Pat. No. 5,109,239 is a continuation-in-part of '270]. In U.S. Pat. No. 5,404,020, filed in 1993 Cobbs teaches a PHASE PLATE DESIGN FOR ALIGNING MULTIPLE INKJET CARTRIDGES BY SCANNING A REFERENCE PATTERN. In U.S. Pat. No. 5,448,269, filed in 1993 by Beauchamp et al., MULTIPLE INKJET CARTRIDGE ALIGNMENT FOR BIDIRECTIONAL PRINTING BY SCANNING A REFERENCE PATTERN is shown. In U.S. Pat. No. 5,835,108, filed in 1996, Beauchamp et al. teach a CALIBRATION TECHNIQUE FOR MISDIRECTED INKJET PRINTHEAD NOZZLES. Each of the aforementioned patents is assigned to the common assignee herein and incorporated herein by reference.
As thermal ink-jet pens are used, damage may occur, such as due to a printhead crash against the adjacent paper, resistor burn-out, ink cogation, and the like as is known to those skilled in the art, causing drop characteristic changes and trajectory changes. Ink drop trajectory can change as a print is being rendered due to ink puddling around the nozzle orifice. Frequent servicing of the printhead, such as by spitting into a waste ink collector or wiping at a service station, degrades throughput. Moreover such wiping of the printhead can wear the nozzle plate which can cause trajectory errors. Thus, while pen "health" is a constant concern, optimally, a pen should only be serviced if and when it is required.
Other techniques related to the actual pixel printing, such as error diffusion, resolution synthesis, or other printing mode digital manipulation are also employed to reduce the number or visibility of print artifacts.
No technique appears to be available for exact printing plane ink drop trajectory determination during printing. Therefore, a method and apparatus is needed to verify each nozzle operation during a print job without impacting the speed of the print job. The method and apparatus should characterize the entire pen swath height in one or two passes.